Covalently immobilized pyridinium ionic liquids on renewable chitosan for mild-pressure CO₂ cycloaddition

Chitosan-supported pyridinium ionic-liquid catalysts were prepared. • Halide, DD, and ionic-liquid loading effects were systematically studied. • Br-IL2Cs-90DD gave 95% cyclic carbonate yield at 0.5 MPa CO 2 . • The catalyst retained high activity over five consecutive cycles. • DFT revealed cooperative activation by N⁺, Br⁻, and chitosan –OH sites. Developing recyclable heterogeneous organocatalysts with clearly identifiable cooperative active sites is important for the mild and solvent-free cycloaddition of CO2 with epoxides. Herein, a chitosan-supported pyridinium ionic-liquid catalyst was constructed by covalently immobilizing bifunctional ionic-liquid moieties onto renewable chitosan through Schiff-base chemistry. The catalyst integrates pyridinium cations, Br⁻ nucleophiles, Schiff-base linkages, and chitosan hydroxyl groups into a cooperative heterogeneous catalytic environment. Among the investigated structural variables, the bromide-containing catalyst prepared from high-deacetylation chitosan (Br-IL2Cs-90DD) showed the best performance, affording 96% epichlorohydrin conversion, 99% cyclic carbonate selectivity, and 95% cyclic carbonate yield at 80 °C and an initial CO 2 pressure of 0.5 MPa within 4 h under solvent-free conditions. This performance is competitive with representative heterogeneous ionic-liquid and biomass-supported catalysts operating under mild-pressure conditions. The recovered catalyst retained 94% conversion, 98% selectivity, and 93% yield after five consecutive cycles, and post-reaction FT-IR/XPS characterization further supported the structural retention of the immobilized ionic-liquid catalyst. Characterization and density functional theory calculations support the proposed cooperative catalytic mechanism, in which the pyridinium cation and Br⁻ promote epoxide activation and ring opening, while chitosan hydroxyl groups help stabilize alkoxide and carbonate-related species. This work provides a biomass-derived immobilized ionic-liquid catalyst for metal-free CO 2 -to-carbonate conversion and offers molecular-level insight into cooperative active-site design. Download: Download high-res image (271KB) Download: Download full-size image